This invention relates to enhanced oil recovery. More specifically, this invention relates to a process of enhancing oil recovery with a cyclic steam injection.
Many hydrocarbons are too thick to be recovered from subterranean petroleum containing formations without assistance. These hydrocarbons are the virgin or residual heavy oils in the reservoir. These heavy hydrocarbons can be recovered through the use of steam drives which heat the formation, lower the viscosity of the hydrocarbons, and enhance the flow of the hydrocarbons from an injection well towards a production well.
However, not all formations are adaptable to a standard injection well-production well type of recovery operation. These types of formations can be subjected to cyclic steam stimulation. However, the steam tends to finger out into the formation volume surrounding the well. This limits the potential volume for enhanced oil recovery because the primary mechanisms for cyclic steam stimulation are viscosity reduction of the in-place oil and gravity drainage back to the injection well during the production cycle. Thus, the total amount of the formation that is reached by the steam injection is limited unless the steam can be forced away from those portions of the formation containing only residual oil saturation.
Residual oil saturation is defined as the immobile oil remaining in that portion of the reservoir swept by steam. Blocking the residually oil saturated portion of the formation from the preferential path of the steam injection would greatly increase the amount of recovered oil. However, any additives such as surfactants, must be able to penetrate the formation but not interfere with the recovery mechanisms of gravity drainage or viscosity reduction. Preferably, these additives should form a penetrating foam which preferentially moves into the residually oil saturated portion of the formation and permits the steam and water portion of the injected compositon to move into the portion of the formation containing recoverable hydrocarbons. Furthermore, any additive contained in the water phase portion of the injection composition should enhance the mobility of the remaining oil.
U.S. Pat. Nos. 3,463,231 and 3,953,338 teach the use of monomer and oligomers of materials referred to as alpha-olefin sulfonates as wellbore cleanout materials. They contain no teachings that the materials can be used in cyclic steaming operations. U.S. Pat. No. 4,393,937 suggests the use of alpha-olefin sulfonate monomers in an injection well-production well combination recovery process. However, the recited patents do not teach or appreciate or suggest any possibility of enhancing oil recovery in cyclic steam operations through the use of alpha-olefin sulfonate oligomers and preferably alpha-olefin sulfonate dimers.
To maximize the extraction of hydrocarbons from a formation, it would be highly desirable to have superior foaming surfactants which inhibit the flow of steam in a steam zone containing only residual oil saturation without inhibiting the backflow of hydrocarbons towards the injection well when it is placed in the production well mode of operation. Furthermore, it would be desirable to have the surfactants in a steam foam recovery process which inhibit the flow of the steam in the zone which contains only the residual oil and enhance the flow of steam in the zone which contains the recoverable hydrocarbons. However, it is desirable not to have the flow of the recoverable hydrocarbons inhibited from returning towards the injection well when it is operated in the production well mode of operation.